Box2dLights

Some how light and light behavior is always fascinated me. Long time ago I noticed that I can do some 2d lights with textures and blendfuncs. Basic model was something like this.
Render scene.
Clear alpha channel.
Render lights to alpha channel with glBlendFunc(GL_SRC_ALPHA, GL_ONE)
Render fullscreen quad with glBlendFunc(GL_ONE, GL_DST_ALPHA)
This give lights and shadows that blend together nicely. But this way you can't get dynamic shadows.

When I coded Boxtrix I did get idea that was too crazy to work. Everyone said that it's not gonna work but I still wanted to test it. Plan was to simulate lights rays using box2d raycasting. After couple hours first prototype was working and looked really ugly but tech seemed to work great.

After Boxtrix was finished I noticed that raycasting lights have really great api and usage is simple. User can add just lights and then one line of code to render lights. That was amazing allready but there was tons of features at wishlist. Softshadows, gles2.0 support, way to query if something is in shadows/lights, lights filters etc. Biggest problem was that usually you don't have alpha channel at render target when you are coding against android. Luckily gl es2.0 solved almoust every problem.
Rendering pipeline goes in box2dLights with gl es 2.0 pretty much like this.

Render scene.
Construct light volumes using crazy amount of raycasts.
Render light volumes to small fbo.
Blur fbo. Using really fast two pass shader that use separate convolution and hardware bilinear filtering tricks. 10samples for 9x9 kernel.
Render lightmap over scene using custom shader that render shadows and lights in one pass.

Box2dLights is complex and powerfull beast but using it is still really simple. Least half dozed published games  is using it and more to come. It's opensource and license is compatible with LibGDX license so it can be used for commercial projects. http://code.google.com/p/box2dlights/

Performance is better than any full gpu solution that I have tested and most of mobile games is fragment shader limited anyway so it's good to use CPU to share some heavy lifting. More info at google code page.